Recently, various imaging instruments are developed, and optical glass constituting lenses suitable for them also has been investigated. Especially, amid downsizing, weight saving and cost-reducing of optical instruments including digital cameras, aspheric lenses permitted aberration correction with fewer lenses are growing in importance.
However, there was a conventional problem for aspheric lenses to have difficulty in performing polishing processes because of their complicated shapes. Thus, processes called mold press molding have been employed, in which glass materials (preforms) are poured into a mold and heated, and pressed after softening.
However, since in mold press molding, preforms are treated after softening, quality control and manufacturing thereof become more difficult as the glass-transition temperature (Tg) of raw materials are higher. Specifically, degradation of a mold, prolongation of processing time and the like are included in the problem. Further, recently, hot-molding of preforms (technology for molding in a direct way by casting melted glass into a mold) is also employed. In such a hot-molding, in case that the liquid phase temperature (Tl) of glass is high, devitrification tend to occur during molding, and therefore, manufacturing of preforms becomes difficult. Thus, glass that have a relatively low glass-transition temperature or a relatively low liquid phase temperature is usually used as optical glass for mold press molding like this.
For example, in patent document 1, optical glass for mold press molding is described. However, since conventional optical glass contains a large amount of ZnO, in order to reduce the glass-transition temperature, it is not sufficient concerning low dispersibility which is important for chromatic aberration correction.